This invention relates generally to inspection of nuclear reactors, and more particularly to ultrasonic examination of welds within a nuclear reactor pressure vessel (RPV).
A typical boiling water reactor (BWR) includes a RPV containing a nuclear fuel core immersed in circulating coolant water which removes heat from the nuclear fuel. The water is boiled to generate steam for driving a steam turbine-generator for generating electric power. The steam is then condensed and the water is returned to the pressure vessel in a closed loop system. Piping circuits carry steam to the turbines and carry re-circulated water or feed-water back to the RPV that contains the nuclear fuel. BWRs have numerous piping systems, and such piping systems are utilized, for example, to transport water throughout the RPV. For example, core spray piping is used to deliver water from outside the RPV to core spargers inside the RPV and to cool the core. Typically, the core spray piping is coupled to a thermal sleeve that is welded to a RPV nozzle and a safe-end is welded to the nozzle.
Stress corrosion cracking (SCC) is a known phenomenon that may occur in reactor components, such as structural members, piping, fasteners, and welds. The reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to SCC. Reactor internal piping, such as thermal sleeves and core spray lines, occasionally require replacement as a result of SCC, the replacement may require welding a new pipe member onto an old pipe member.
Some known methods of inspecting welds for SCC utilize a phased array probe. Known phased arrays include a group of transducer elements used together with relative time, or phase shifts between the elements. The combined elements act as a single instrument that can be steered to distinct points in space. Typically, known methods average ten hours to scan a thirty-centimeter weld.